Polycarbonate (PC)/acrylonitrile–butadiene–styrene terpolymer (ABS)/montmorillonite (MMT) nanocomposites have been obtained by melt compounding in a twin-screw extruder. Structural and elastic properties have been characterized. The theoretical analysis of the elastic properties of PC/ABS/MMT composite with a widely varied number of elementary layers in clay filler particles has been performed in order to estimate the influence of the degree of exfoliation of particles on the effect of reinforcement of the material. Results of the theoretical analysis have been compared with the experimental data about the effects of unmodified and two types of organically modified montmorillonite clays on the elastic characteristics of the investigated PC/ABS nanocomposites. It has been demonstrated that addition of dimethyl benzylhydrogenated tallow ammonium treated MMT allows effectively increase modulus of elasticity of PC/ABS composites at minor nanofiller content (up to 1.5 wt.%) due to the development of the nanocomposites with high degree of exfoliation of the clay particles. Besides it has been observed that addition of the nanoclay causes glass transition temperatures of the polymeric components of the investigated multicomponent system to approach each other that is believed to testify about the improvement of compatibility between the ABS and PC phases.